Sometimes something is unresolved where you expect somebody else to resolve it. Maybe a perceived injustice has been done, somebody has done something bad and needs to atone for it, etc. Forgiveness is a change in the state of mind to where you no longer feel this matter to be unresolved: it is resolved in your mind, even though the expected resolution may not have happened. Forgiveness gives you your peace of mind back. To the person that is forgiven, forgiveness is a great relief, and in some cases actually opens the door to atoning actions.